Spec out an LED

rocky79

Newly Enlightened
Joined
Nov 12, 2009
Messages
5
Hello,

I am trying to design a light stick that produces 120 lumens at a distance of 38" away. The light stick will have 20 to 30 leds.

How do i go about finding the lumens specified on a typical led datasheet per led?

Thanks
 

Benson

Flashlight Enthusiast
Joined
Feb 15, 2009
Messages
1,145
You don't produce lumens at a distance. Lumens are a measure of the total light output (which stays the same, just gets "stretched thinner" over an expanding spherical wavefront as the light travels outward, candelas are the intensity in a particular direction, and lux are the illuminance, or amount of light hitting per unit area, which drops off with distance (inverse-squarely, for a point-source).

Since you're talking about getting ~100 lumens from more than one or two LEDs, I assume you're looking at 5mm or similar LEDs -- the specs on these usually omit the total flux (lumens) in favor of centerline intensity (candelas) and beam width (degrees). So you just need to figure out what illuminance you want, and what area, and you should be there -- intensity and illuminance are related as 1 cd makes 1 lux at 1 meter.

If you go with power LEDs, however, they're often given as lumens and a beam profile of relative intensity vs. angle -- by integrating over the hemisphere, you can get the centerline intensity, and then the relative intensity scale tells you exact intensity in any direction. But this is a major pain to describe, and if you know _how_ to integrate it, you'll know how to derive the formula. ;)
 
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